An organic light emitting display device including: an optical sensor for generating an optical sensor signal corresponding to brightness of ambient light; a first luminance control unit for generating a pulse width (EW1) of a brightness control signal in accordance with the optical sensor signal (Ssel) and supplying a selection signal; a second luminance control unit for generating a fluctuated value (EW2) for the pulse width of the brightness control signal in accordance with data of one frame; and a luminance control signal generation unit for generating a luminance control signal (Vc) for controlling a scan driver in accordance with the pulse width of the brightness control signal and the fluctuated value. Here, the second luminance control unit is turned on/off according to the selection signal from the first luminance control unit.